1. Field of the Invention
The present invention relates to a process for producing a photoconductive, semiconductive, or insulating silicon-containing deposit film on a given substrate under ordinary pressure by utilizing thermal energy. More specifically, the invention relates to a process for producing a deposit film of silicon, particularly amorphous silicon (hereinafter designated as a-Si), on a given substrate under ordinary pressure through thermal decomposition of a gas containing mainly of a cyclic silane.
2. Description of the Prior Art
It is generally known that SiH.sub.4 is decomposed with exciting energy such as electric discharge energy or thermal energy to form a-Si deposit films on substrates, and that the products are utilized for various purposes.
However, this process for producing a-Si deposit films from SiH.sub.4 involves the following drawbacks:
(1) The film forming speed needs to be lowered for securing good product quality.
(2) Heating at a high temperature of 500.degree.-600.degree. C. is necessary to decompose SiH.sub.4. This matter restricts the choice of substrates and other production conditions.
(3) For producing large area, thick deposit films, there are difficulties in securing the electrical and optical uniformity of the product and the constancy of product quality, and irregularities on the surface of the deposit film and defects in the interior thereof are liable to result.
(4) Because of the high temperature of film formation, little hydrogen atoms, which play a role important to the a-Si film quality, are incorporated into the film and a high density of localization levels results. This matter exerts undesirable effects on film properties suitable for semiconductor use.
Recently a process for producing a-Si deposit films from Si.sub.2 H.sub.6 has been proposed to offset the above-mentioned drawbacks. This process, though drawing attention, is still unsatisfactory particularly in that a high temperature of at least 400.degree. C. is necessary to utilize thermal energy for the decomposition of Si.sub.2 H.sub.6. The reduction of energy consumption or improvement of productivity in this process is the problem to be solved in the future. While Si.sub.2 H.sub.6 readily decomposes to SiH.sub.4 and excited fragments and therefore is used more advantageously than SiH.sub.4, the main decomposition product SiH.sub.4 needs to be decomposed successively in order to utilize Si.sub.2 H.sub.6 effectively. Accordingly, striking improvement in the efficiency of Si.sub.2 H.sub.6 decomposition can be scarcely expected.
In addition, Si.sub.2 H.sub.6 is expensive since it is not a generally available material and its production process has not been established. In consequence, the commercialization of the Si.sub.2 H.sub.6 -employing process is economically difficult unless effective formation of the deposit film is achieved.
U.S. Pat. No. 4,363,828 describes the possibility of utilizing higher silanes than Si.sub.2 H.sub.6 but none of embodiments referring to cyclic silanes and measures of solving the above noted problems. Additionally, it has not been reported that such a higher silane could be produced in high yields or a deposit film therefrom was used as a valuable product. For industrializing the production of deposit films from a higher silane, many subjects remain in respect to the development of techniques for producing and utilizing the higher silane.
If the production of a-Si deposit films at a low energy level becomes feasible, the product films are expected to have high quality with uniformity, the control of production conditions will be facilitated, and the industrial productivity as well as the reproducibility will be improved.
The prior art production of a-Si deposit films is also carried out under reduced pressure in most cases. This is accompanied by the rise in costs of equipment and costs for maintaining and supervising equipment. Hence, improvements in this respect are desired now from the economical and technical points of view.
In the prior art process for producing a-Si deposit films, electric discharge energy is generally used to form the films. This inevitably leads to high costs of equipment, particularly of the reactor, and necessitates the exercise of great care on the control of production conditions so as to secure the uniformity, constancy, and reproducibility of product quality for the formation of large area, thick deposit films. Thus, there are various industrial problems in the present state of the art.
Also in the deposition process employing thermal energy for the decomposition, a high temperature of at least 600.degree. C. is necessary though this depends upon the kind of gas used. Such a high temperature restricts substrates usable for the deposition and tends to eliminate important hydrogen atoms bound to the intended a-Si, so that desirable characteristics of products are difficult to obtain. Additionally the high temperature process is unsuited for mass production because of the low decomposition efficiency and deposition rate.